5.7 HEMI & 87 octane gas

[Chase300]

Originally Posted by Inferno07

Keven,

Well I didn't really care to go into all the details, of course if you want to we certainly can. I built my first 11 second on street tire Mopar when I was 16. I do know a little bit about engines (Hemi's and Wedges and a few others). There are many many more factors than even what you listed. But a general rule of thumb is that a sub 10:1 static compression engine (with alloy heads no less) is not likely to detonate. Add to that the fact that alloy heads do not hot spot like cast iron and then add to the fact that it's a hemi which has much larger combustion chambers with less sharp edges to retain hot spots than your typical wedge, less cause for detonation.

And then of course we can go into the whole PCM controlls the ignition timing, knock sensors and all the other technological whiz bang that makes the modern multipoint efi system work... It's not going to detonate on 87 and I'm not "cheap" for using it. There's a time and a place for premium and it ain't now...

Ya know.. This is my second "new" hemi (and certainly not the first "hemi" in the family considering pops was a two time Nat record holder with a 68 SS/AA Hemi Cuda one of 50 ever made) I tow a 9K boat with an 04 non MDS Hemi Ram on 87 and it's never suffered from detonation.

And I do agree, stick with what the manufacturer recommends and they say for the 5.7 that 87 is just fine.

And I think this is a great topic by the way!

Right On! Agree 100%!
No need that I've seen to run 89, at least not here in Atlanta as the 87 octane fuel sold here is fine. Maybe elsewhere 87 can be 86,85 or lower in which you would have pinging, but here 87 works as well as 89 and actually as long as there is no pinging or knock, its a better choice. You always want to run the lowest octane possible....more power that way.
Oh yeah, the manual says its OK.

[mystere485]

But there is no way possible to AVERAGE 30 mpg with a 300C for more than a mile or 2. It just takes a certain amount of energy to move a 4200 lb brick down the road, no matter what grade gas you use. Technology is fantastic but it can't repeal the laws of physics.[/quote]


You know, that may be true in your part of the country. To get to the next largest city from where i live is roughly 150km's. @100kph on the 35km stretch to the main highway I average 26mpg because of a couple uphill's along the way. once i make it to the 4 lane yellowhead highway I can average @105kph 28mpg in -10 degree weather and recently at +4 i managed it at just shy of 30mpg @100kph which im sad to say is 10kph below the speed limit. Myself i am 260lbs and my gf(god bless my soul) is around the same give or take a few pounds. My car is a 2005 model with now 72,000 km's. I have yet to switch to synthetic as i have had the car less than a month.

[DustyButt]

Originally Posted by mystere485

But there is no way possible to AVERAGE 30 mpg with a 300C for more than a mile or 2. It just takes a certain amount of energy to move a 4200 lb brick down the road, no matter what grade gas you use. Technology is fantastic but it can't repeal the laws of physics.

I've gotta disagree. I actually hit 30.4 mpg for a long stretch while driving from NYC to Baltimore last spring. On top of that I switched from 89 to 87 out of curiosity in DE and I never averaged above 27 mpg.

On top of that my car felt rougher and more sluggish with 87 when I floored it from low revs.

I know... I know... I heard it all through flight school "don't trust the seat of your pants", but I'm just saying what I felt.

But the mpg #'s don't lie.

Just my 2 cents.

[GoofyTimL]

Originally Posted by chuckkent

In Ohio the land where I live is very flat. I averaged 30.8 for 15 miles by feathering the gas pedal....... I average 26 to 28 on the highway in cruise at 60 and 19 to 21 around town.....One jack rabbit start and you can for get it but the fun of acceleration off sets the mpg every now and then.......

Chuck... the best mileage I've ever had with the H.E. was 26.4 mpg going 400 miles across the mountains of Utah and the continental divide in Colorado... using 89 octane and a fuel additive. Read that as a lot of miles going downhill steeply with the instantaneous mpg at 40+ mpg.

On the flats of Ohio, which I've crossed numerous time enroute to Chicago and MN, the best my H.E. can do is about 23-24 mpg. Our local driving nets 15.5-16.5 regularly with a mixture of 50:50 Sam's Club 87 and 91 octane, plus the additive (ACES IV Fuel Catalyst).

If you're getting 27-30 mpg with 87 octane "feathering" the gas pedal on the road and nearly 20 in local driving, I'd say you've got one magic machine and I wouldn't mod a thing on it! Congrats!

[chuckkent]

My wife was with me when I did these tests. I traded a Sebring convertible and a Jeep Liberty to get this car. The previous owner had just bought a new 2008 H.E. identical to this 2006 and he told me in the showroom you won't believe the gas mileage I get with that car. He is a rich farmer from around here that owns a winter home in Florida. He said on the trip he regularly gets 28 and 19 or better around town..... There are two qualifiers... one I'm retired and drive like an old lady and the other thing all towns around here are 50K or less so its not like I have to idle in traffic in Chicago or NY........I use 87 cheapest gas I can find and have the oil changed by the dealer because for the money the quality and service can't be beat!!!!!!

[HKS]

I get better nileage with 87 than with 89. 89 has 10% alcohol in it. The manual says octane 89 and 87 acceptable.

HKS

300c
2005 COTY

Originally Posted by Mike_Levy

You may not be realizing it, but the knock sensors are pulling timing to compensate for you being cheap and saving 30 cents a tank. At least you better hope they are, other wise you'll be spending a helluva lot more then you "think" you're saving on a rebuild or replacement.

I don't understand the logic of not putting in good gas. You spent $30k on a car. Even if you filled up the tank every single day, you'd barely save a hundred dollars a year, using 87 over 89. Whats the point?

[Easy]

i use 93 Octane ... last alot longer for me than the regular crap...

[Chase300]

If someone has a Predator they can hook it up, set to record and see if any knock happens running 87. If no knock, then no power loss and no reason to run 89.

[chuckkent]

Alcohol in higher octane gas is something to consider as far as gas mileage. Alcohol has a lot less calories in it than gas so a 10% mix will hurt gas mileage probably up to 5 miles per gallon. For us 5.7L users 87 is fine but for the 6.1 crowd higher octane is justified.

[magnuman]

There are too many variables involved to say "one octane fits all." The manual says running 87 in a 5.7 is acceptable. If you're in an area that has "doctored" fuel (ethenol/oxygenated, etc.) you may have more of a problem, especially in the mileage department. If you're at altitude and a cooler climate, like I am, you can easily get away with a lesser octane, especially in the winter time. Some places (about 5000 ft. and above) you could probably get away with 85. I've run it on I-80 between Winnemucca, NV. and Salt Lake with no problems. IMO, the best way to determine the "correct" octane for your vehicle is to monitor knock retard under various conditions. I do this routinely with the Dashhawk, and I can say that, in town (as long as you're not always hammering it) you can easily get away with a lesser octane. I have one screen setup with a realtime graph of timing and knock retard. Personally, in the winter, most of the time I run 87, and in the hot summer months (100-110) I'll run 89 or 91. If I go to the races, I'll switch to 93 about a week before. Currently I am running a modified Superchips 91"stock" tune on the Diablosport. This configuration, at least in the winter, seems to work best for me, and it also gives back full use of ESP, which the DS by itself makes much less sensitive.